Evolutionary characteristics of sealing capacity of deep salt caprocks under temperature-pressure coupling

Three sets of experiments, including constant temperature-variable-pressure, variable temperature-constant pressure, and temperature-pressure coupling, were executed to simulate the influence of temperature and pressure conditions on the evolution of salt rock cap sealing ability using the experimental device of high temperature and high pressure triaxial rock mechanics testing system on salt rock samples from Jintan Salt Cave, China. The results indicate that: ① The coupling effect of temperature and pressure leads to a significant reduction in short-term strength, significant enhancement of plasticity of salt rock, and shortening of brittle-to-plastic transition time. The percentage (T_i) of short-term strength reduction of salt rock caused by temperature is greater than 35.06 % while the maximum shortterm strength reduction percentage (P_i) of salt rock caused by pressure is only 38.25 %, indicating that the mechanical properties of salt rock are mainly controlled by temperature during the burial process. ② Under a single pressure, salt rock fractures have a trend of gradually decreasing and healing; while under the coupling of temperature and pressure, the salt rock will experience a phenomenon of “re-cracking”, with pressure dominating the low temperature stage (temperature < 90 ℃ ) and temperature dominating the high temperature stage (temperature ≥ 90 ℃ ), resulting in creep damage to the salt rock. ③ An evolutionary model of “brittleness-brittleness-plasticity-plasticity-creep damage-damage healing” was established for deep salt rocks. Under the coupling of temperature and pressure, the creep damage stage is extremely unfavorable for the preservation of oil and gas reservoirs under high temperature conditions. Therefore, accurately determining the evolution characteristics of salt rock sealing ability can provide a theoretical basis for analyzing the enrichment patterns of deep pre-salt oil and gas reservoirs.